Introduction to Windows PowerShell’s Switch Case
As with other scripting languages, PowerShell provides a variety of commands to perform branching logic. For simple cases, with few options, the ‘If’ construction works well. The difficulty arises when ‘If’ becomes a victim of its own success and you have 5 or 6 options, for that situation it is more efficient to use PowerShell’s ‘Switch’ command.
Topics for PowerShell’s Switch Command
- The Basic Structure of PowerShell’s Switch Statement
- Case Study 1 – WMI Disk
- Case Study 2 – Switch with -Wildcard
It is likely that your real-life task for Switch will be trickier than the following simple examples. However, it is worth studying a range of basic examples to get a feel for the structure and the rhythm of the command.
The Basic Structure of PowerShell’s Switch Statement
A curious feature of the construction is that the word ‘Switch’ introduces the input, and is then never seen again, all that you see thereafter is rows of patterns with their matching {Statement Blocks}. Also, observe that there is an extra pair of {braces} surrounding the whole pattern section.
Switch (pipeline) {
Pattern 1 {Statement block}
Pattern 2 {Statement block}
Pattern n {Statement block}
}
For simple examples, you could write the Switch command all on one line:
Switch (3) { 1{ "Red" } 2{ "Yellow" } 3{ "Green" } }
Result: Green. PowerShell switches the input of 3 to an output of Green. Incidentally, the equivalent of ‘Switch’ in VBScript is ‘Select Case’.
Learning Points
Note 1: Trace the overall structure of the Switch command:
Switch (Value, or Pipeline in parenthesis) {Actions in braces}
Note 2: The “Block” for each Switch is enclosed not only by {braces}, but also by speech marks { “Yellow” }.
Note 3: Remember to pair the initial { brace, with a final matching brace, even if the whole structure spans multiple lines.}
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Case Study 1: Switch with WMI Disk
A difficulty occurred when we interrogated the computer’s disks with WMI. Specifically, the output reports DriveType as a number, whereas we want a meaningful name for the type of disk. The extra step in this example was to research which drive type corresponded to which number, for example if $Drive.DeviceID returns a value of 3, that means the disk type is a “Hard drive”.
Case Study 1 – Problem understanding the output
# Case Study 1 - Problem
$Disk = Get-WmiObject win32_logicaldisk
Foreach ($Drive in $Disk) {
$Drive.DeviceID + $Drive.DriveType
}
We want an answer to the question: ‘What does the DriveType number mean?’ It would be useful if the script gave us a name rather than a number. Research reveals that there are at least 5 possible disk types, therefore multiple ‘If’ statements would be cumbersome, ‘Switch’ is more elegant.
Case Study 1 – Solution use ‘Switch’
Whenever I add a ‘Switch’ statement to my script, I think – ‘good job’. And I follow up by saying: ‘Why don’t I use the Switch construction more often?’
# Case Study 1 - Solution with PowerShell 'Switch' Param
Clear-Host
$Disk = Get-WmiObject win32_logicaldisk
Foreach ($Drive in $Disk) {Switch ($Drive.DriveType) {
1{ $Drive.DeviceID + " Unknown" }
2{ $Drive.DeviceID + " Floppy or Removable Drive" }
3{ $Drive.DeviceID + " Hard Drive" }
4{ $Drive.DeviceID + " Network Drive" }
5{ $Drive.DeviceID + " CD" }
6{ $Drive.DeviceID + " RAM Disk" }
}
}
Learning Points
Note 4: Before examining the solution in Case Study 2, take the time to understand the underlying WmiObject construction in Case Study 1. In particular observe the format:
Foreach (condition) {Block Command}.
Note 5: The case study solution builds on the first script by adding the Switch block. To my way of thinking, there are two loops, the outer Foreach and an inner Switch loop. Check what I mean by matching the last two {lonely} braces with their opening counterparts.
Challenges
Challenge 1: Just to get experience and control of the script try changing “Hard Drive” to “Fixed Disk”
Challenge 2: Create a mapped network drive, then run the script again. Launch Windows Explorer then click on the Tools menu, this is the easiest way to map a local drive letter to a UNC share.
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Learning More About PowerShell’s Switch Statement
For once PowerShell’s help did not do what I wanted, Get-Help Switch did not work. However, I discovered that the secret was to prefix switch with about_. Try:
Get-Help about_switch
This help file introduced me to the -Wildcard statement, which helped with matching the input to the Switch clause.
Incidentally, PowerShell supports a whole family of about_ commands, for example you could try:
help about_Foreach. See more about PowerShell’s Help About_ family.
Switch Case Study 2 – Featuring Switch with -Wildcard
The objective of this ‘Switch’ example is to return a count for each type of error message in the Application eventlog.
# PowerShell Switch script to count eventlog messages
Function EventType{
BEGIN {
$errors = 0
$warnings = 0
$info = 0
}
Process {
Switch -Wildcard ($_.entrytype) {
"err*" {$errors++}
"warn*" {$warnings++}
"info*" {$info++}
default {}
}# End of switch block
}# End of Process block
END {
Clear-Host
"The Application log contains " + $errors + " error messages."
"The Application log contains " + $warnings + " warning messages."
"The Application log contains " + $info + " information messages. `n"
Get-Date
} # End block
} # End of function
Get-eventlog "application" -newest 250 | EventType
Note 6: Observe the -Wildcard switch with err*, info* and warn*. To test the effect, try removing the -Wildcard statement.
Note 7: The layout above emphasises the branches, or the multiple ‘Patterns’, whose values get switched to their respective {Blocks}.
See more about PowerShell parameter functions »
Summary of PowerShell’s ‘Switch’ Statement
The ‘If’ family are easy to use for scripts that require branching logic. However, when the number of options exceeds about 5, then ‘Switch’ is easier to code and easier to add more options. By trying a few simple examples you will soon appreciate the types of bracket, and the structure of the pattern with its matching statement block.
I say again, ‘Switch’ is one of the most satisfying constructions to create, therefore never miss a chance to replace multiple ‘If’s with one ‘Switch’.
See more Windows PowerShell flow control examples
• PowerShell Switch Statement • PowerShell Real-life Techniques • Free Permissions Analyzer
• Differences between For, ForEach and ForEach-Object • PowerShell Loops • PowerShell Home
• Conditional Operators • Do While Loop • PowerShell If Statement • PowerShell Brackets
Please email me if you have a better example script. Also please report any factual mistakes, grammatical errors or broken links, I will be happy to correct the fault.
